1. Field of the Invention
This invention relates to a process for foaming and sulfur-curing blends of an ethylene/vinyl ester/carbon monoxide or ethylene/vinyl ester/sulfur dioxide copolymer with a polyvinyl or polyvinylidene halide, such as, for example, polyvinyl chloride or polyvinylidene chloride.
2. Discussion of Related Art
It is known to foam and cure polymers and polymer blends such as, for example, ethylene/vinyl acetate/carbon monoxide terpolymers or blends of such terpolymers with polyvinyl chloride; see, U.S. Pat. Nos. 4,370,423 and 4,391,923, both to Rys-Sikora. Curing of the foamable polymer compositions of those patents is accomplished by means of free radicals. Those compositions, therefore, contain free radical generators, such as peroxides, peresters, or azides.
It also is known to foam and sulfur-cure diene elastomers such as, for example, natural rubber, SBR, and similar materials. Foams made by such processes are readily available commercially.
Further, it is known to sulfur-cure ethylene/vinyl acetate/carbon monoxide (E/VA/CO) terpolymers, as described in U.S. Pat. No. 4,172,939 to Hoh.
Foaming and curing polymer blends in the manner described in the above-cited Rys-Sikora U.S. Pat. No. 4,391,923 has certain shortcomings. Firstly, free radicals are sensitive to, and are destroyed by, oxygen. As a result, foaming and curing cannot be carried out in the presence of air, for example, in an oven, but normally would be carried out by compression molding, in the absence of air. This, in turn, makes it impossible to produce cured foams by a continuous process, where the composition is at the same time foamed and cured in an oven or a heat tunnel. A second drawback of the free-radical cures is that free radicals are easily consumed by various conventional compounding ingredients, such as, for example, antioxidants, certain extending oils, and plasticizers, and thus quickly become depleted, leaving the composition uncured or only partially cured. Such conventional compounding ingredients are, therefore, often omitted from those compositions.
It is, therefore, desirable to be able to produce foams of such polymer blends by a process which would not suffer from those limitations.